The present invention relates to a control method and a control system for an electric vehicle having an inverter for converting a direct current electric power such as an electric power of a battery into an alternating current electric power source and a synchronous motor as the driving force of the vehicle, and more particularly relates to a control method and a control system for an electric vehicle having a permanent-magnet type synchronous motor as the driving force of the vehicle.
Development of permanent-magnet type synchronous motors for driving an electric vehicle has been progressing. The efficiency of the permanent-magnet type synchronous motor can be increased in a low speed and low torque region compared to an induction motor. An electric vehicle driving system using such a permanent-magnet type synchronous motor is disclosed, for example, in Japanese Patent Laid Open Application No. 6-315201.
In an electric vehicle, for example, an electric automobile, there are some cases in which the motor may be driven at a high speed above a maximum controllable rotating speed during a coasting operation on a descending road.
In a case of controlling a synchronous motor generally used in the industrial field, when a speed of the motor is increased and exceeds the controllable range, the motor is controlled by being judged as to be in an abnormal condition. In a case of an electric vehicle, the motor is controlled by repeating to drive it up to a maximum allowable speed for the power-train system and then to operate it in a normal re-operation within the controllable range as the speed of the motor is decreased. When the motor is driven at a speed above the maximum controllable speed, a protective relay (hereinafter referred to as an inverter relay) provided between an inverter and a battery is operated. That is, it can be considered that the inverter relay is disconnected to protect the power train control system of the electric vehicle when the motor is driven at a speed above the maximum controllable speed.
The inverter relay is for connecting or disconnecting the inverter and a smoothing capacitor to and from the battery, and is connected and disconnected according to a key switch IGN signal.
In the electric vehicle power train system, the inverter relay is disconnected under the following conditions during driving.
(1) Speed of the motor exceeds the controllable speed,
(2) the key switch IGN is in OFF state, or
(3) an abnormal condition occurs anywhere except for in the inverter (an abnormal condition capable of re-power-driving).
After that, it is necessary that the inverter relay is re-closed during running in order to re-operate the inverter when the speed of the motor is decreased to a value within the controllable range from the above state (1), the IGN described in the above item (2) is brought in ON state and a re-driving command described in the above item (3) is generated.